The invention relates to a device for treating skin by means of radiation pulses, which device comprises a housing, a radiation source, a control unit for controlling the radiation source, and a detector for detecting an image of at least a part of the skin, said control unit comprising a processor which, in operation, determines from the image detected by the detector a dimension of a target object to be treated on the skin and which determines from said dimension a pulse dose of the radiation source that is necessary to treat said target object.
A device of the type mentioned in the opening paragraph is known from EP-A-1 031 324. The known device is an epilation device. The radiation source used in the known device is a laser source whose wavelength is readily absorbed by melanin, which is a pigment that is present in large concentrations in hairs and hair roots. The device further comprises a laser beam manipulator that is equipped with a number of tiltable mirrors by means of which, in operation, a laser beam generated by the laser source is positioned in a target position on the skin to be treated. The detector used in the known device comprises a heat image sensor. In the known device, prior to the epilation process, the skin is exposed to a diffuse laser beam having a comparatively low dose, after which a heat image of the irradiated skin is detected by means of the detector. By virtue of the large concentrations of melanin in the hair roots, said hair roots and the skin directly surrounding the hair roots are heated more strongly by the diffuse laser beam than other parts of the skin, so that the hair roots show as heat spots on the heat image. The processor subsequently determines the positions and thicknesses of the individual hair roots from the positions and dimensions of the heat spots in the heat image. Subsequently, the processor controls the laser beam manipulator in such a manner that the laser beam is present successively in the positions of the hair roots thus determined, the laser source being activated in each one of said positions by the control unit so as to generate a radiation pulse having a comparatively high pulse dose. The pulse dose necessary to destroy a hair root is determined by the control unit for each individual hair root from the thickness of the relevant individual hair root determined from the heat image. As the processor determines the necessary pulse dose for each individual hair root, in this known epilation device the total amount of radiation energy of the laser source that is necessary to remove all hairs is limited, and pulse doses that are too high leading to unnecessary skin irritations or even skin injuries and to an unnecessarily high energy consumption of the epilation device are precluded as much as possible.
A drawback of the known device resides in that the pulse doses of the laser source that are necessary to destroy individual hair roots are often insufficiently accurately determined by the processor. As a result, in many cases too low a pulse dose is applied so that a hair root is not, or only partly, destroyed and the result of the epilation operation is insufficient, or conversely, the pulse dose applied is too high, as a result of which the hair root is destroyed but also part of the skin tissue around the hair root is adversely affected, thereby causing skin irritations.
It is an object of the invention to provide a device of the type mentioned in the opening paragraph, wherein the pulse dose necessary to treat a target object present in or on the skin can be more accurately determined so that pulse doses of the radiation source that are either too high or too low can be precluded more effectively and a more satisfactory result of the treatment with fewer skin irritations is achieved.
To achieve this object, a device of the type mentioned in the opening paragraph is characterized in accordance with the invention in that the processor also determines from the detected image a color of the target object to be treated, the processor determining the necessary pulse dose from said dimension and said color. It has been found that in many cases the pulse dose that is necessary to treat a target object present on or in the skin depends not only on the dimension(s) of the target object but is predominantly determined by said dimension(s) and by the absorption coefficient of the target object for the radiation generated by the radiation source. The necessary pulse dose is generally lower as said absorption coefficient is higher. It has been found that in many cases, such as in the case of hairs and blood vessels, said absorption coefficient is strongly related to the color of the target object. As, in the device in accordance with the invention, the processor determines from the detected image of the skin not only the dimension of each individual target object but also the color of each individual target object, said processor is capable of additionally determining, for example by means of a previously empirically determined relationship between the color and the absorption coefficient of target objects of the type to be treated, the absorption coefficient of each individual target object by means of the detected image. In this manner, the absorption coefficient of each individual target object is determined in a particularly efficient and practical way. As the processor determines the necessary pulse dose from said dimension and said color that is related to the absorption coefficient, the pulse dose necessary to treat each individual target object is accurately determined, so that pulse doses that are too high or too low are more effectively precluded.
A particular embodiment of a device in accordance with the invention is characterized in that the radiation source comprises a laser source and the device is provided with a laser beam manipulator that can be controlled by the control unit, which laser beam manipulator is used to position a laser beam supplied by the laser source in a target position on the skin, the processor determining from the detected image also a position and/or orientation on the skin of the target object and said processor determining the target position from said position and/or orientation, and the control unit activating the laser source if the laser beam manipulator is in a position that corresponds to the target position. As the target position of the laser beam is determined by the control unit in the manner described above, and the laser source is activated as described above by the control unit if the laser beam manipulator is in a position that corresponds to the target position of the laser beam, determination of the target position and activation of the laser source in the target position occur fully automatically, so that the device is safe for use by inexperienced persons and in particular suitable for the consumer market. To determine the target position from the image of the skin detected by the detector, the control unit comprises for example an algorithm which is suitable for this purpose, and by means of which the position and/or orientation of the target object to be treated on or in the skin can be determined on the basis of the image information, and by means of which the target position can be determined on the basis of said position and/or orientation of the target object. In this manner, by means of the detected image, the dimension and the color of the target object to be treated as well as the target position of the laser beam on the skin is determined, so that the detector is used in a practical and efficient manner.
A further embodiment of a device in accordance with the invention is characterized in that the detector comprises a CCD image sensor or a CMOS image sensor. Said detectors have comparatively small dimensions and are very accurate. The CMOS image sensor has the additional advantage that the electronics necessary to read and interpret the detected image can be integrated in the image sensor, thus leading to a simplification of the structure of the device.
Yet another embodiment of a device in accordance with the invention is characterized in that the processor determines the color of the target object to be treated from a gray value that the target object has in the image of the skin detected by means of the image sensor. In this embodiment use is made of a comparatively simple, low-cost image sensor, and the detected image comprises a comparatively simple gray scale distribution. It has been found that in most cases satisfactory results are achieved by using such a simple image sensor.
A particular embodiment of a device in accordance with the invention is characterized in that the device is a hair removing device, wherein the processor determines from the detected image a thickness and a color of a hair to be removed, and the detector determines from said thickness and color a pulse dose that is necessary to remove said hair. In this embodiment of the device in accordance with the invention, the invention becomes effectual in a particular way because the absorption coefficient of hairs for the radiation of the radiation source depends substantially on the color of the hairs. To remove or cut hairs by means of radiation pulses, use is generally made of a laser source having a wavelength that is absorbed well by melanin, which is a natural pigment present in hairs. Dark hairs generally have a comparatively high concentration of melanin, while fair hairs generally have a lower concentration of melanin. As a result, hairs demonstrate a strong relationship between the color and the absorption coefficient for the applied laser radiation, so that the absorption coefficient can be accurately and reliably determined from the color of the hairs. By virtue thereof, the pulse dose of the laser source that is necessary to remove or cut hairs can be determined in an accurate and reliable manner for each individual hair.
A further embodiment of a device in accordance with the invention is characterized in that the device is a shaver, wherein the processor determines the thickness and the color of a hair to be cut close to a position on the hair where the hair projects from the skin. It has been found that the pulse dose that is necessary to cut a hair near the surface of the skin depends mainly on the thickness of the hair near the surface of the skin and on the absorption coefficient/color of the hair. As the processor determines the thickness and the color of the individual hairs in said position near the surface of the skin, the necessary pulse dose for each individual hair is determined in an accurate and reliable manner. As a result, the shaving performance of the device in this embodiment is satisfactory, and skin irritations are limited as much as possible.
A still further embodiment of a device in accordance with the invention is characterized in that the device is an epilation device, wherein the processor determines a thickness of the hair root from the thickness of the hair by means of a predetermined relationship between hair thickness and hair root thickness, and wherein the processor determines the necessary pulse dose from the color and from the hair root thickness thus determined. It has been found that the pulse dose that is necessary to destroy a hair root depends mainly on the thickness of the hair root and on the absorption coefficient/color of the hair. As hairs generally demonstrate a strong relationship between the thickness of the hair and the thickness of the hair root, in this embodiment the thickness of each individual hair root is determined in a practical and reliable manner by the processor from the thickness of the visible portion of the relevant hair, so that the necessary pulse dose for destroying each individual hair is determined also in an accurate and reliable manner by the processor. As a result, the epilation performance of the device in this embodiment is satisfactory, and skin irritations are limited as much as possible.